Saturday, August 25, 2012

Paper finds Arctic sea ice extent 8,000 years ago was less than half of the 'record' low 2007 level

A paper published in Science finds summer Arctic Sea Ice extent during the Holocene Thermal Maximum 8,000 years ago was "less than half of the record low 2007 level." The paper finds a "general buildup of sea ice from ~ 6,000 years before the present" which reached a maximum during the Little Ice Age and "attained its present (year 2000) extent at 4,000 years before the present"

Horizontal axis is number of years before the present. Multiyear sea ice reached a minimum between ~8500-6000 years ago during the Holocene Thermal Maximum (HTM).

Excerpt: In general, our sea-ice record for North Greenland follows the Holocene climate development, with an early warm period followed by declining temperatures, which were punctuated by relatively warmer and colder intervals (17, 25). The reduction of the HTM sea ice in northern Greenland fits with the simulated ice distribution and surface temperature in orbitally forced ECHAM5/JSBACH/MPI-OM (EJM) and LOVECLIM general circulation climate model simulations (3, 4, 10). A tentative first approximation of the large-scale changes associated with the observed ice retreat north of Greenland can be obtained by selecting among the numerical experiments performed with the LOVECLIM model those that are the most similar to our observations [experiments E3 to E5 (3) and fig. S3]. In this exercise, our records would correspond in the model to an Arctic Ocean sea-ice cover in summer at 8 ky B.P. that was less than half of the record low 2007 level. The general buildup of sea ice from ~6 ky B.P. agrees with the LOVECLIM model, showing that summer sea-ice cover, which reached its Holocene maximum during the LIA, attained its present (~2000) extent at ~4 ky B.P. (fig. S3)

ABSTRACT

We present a sea-ice record from northern Greenland covering the past 10,000 years. Multiyear sea ice reached a minimum between ~8500 and 6000 years ago, when the limit of year-round sea ice at the coast of Greenland was located ~1000 kilometers to the north of its present position. The subsequent increase in multiyear sea ice culminated during the past 2500 years and is linked to an increase in ice export from the western Arctic and higher variability of ice-drift routes. When the ice was at its minimum in northern Greenland, it greatly increased at Ellesmere Island to the west. The lack of uniformity in past sea-ice changes, which is probably related to large-scale atmospheric anomalies such as the Arctic Oscillation, is not well reproduced in models. This needs to be further explored, as it is likely to have an impact on predictions of future sea-ice distribution.

18 comments:

Top picture demonstrates that the current century is unusual. There is no other precipitous drop for as long in sea ice coverage in the previous 8000 years - and of course the line after 2000AD would be an even steeper one and bottom out close to the values 8000 years ago.

To my mind the top graph is confirmation that something very unusual is happening in the Arctic, regardless of the fact that Arctic sea ice cover has previously been as low in summer (and even non-existent at other times in the geological record).

The top graph is the output of computer models of the sea ice extent with different assumptions of greenhouse gas forcing. The top black line has the lowest assumed forcing and is probably the closest to reality and shows, as the authors state, "The general buildup of sea ice from ~6 ky B.P. agrees with the LOVECLIM model, showing that summer sea-ice cover, which reached its Holocene maximum during the LIA, attained its present (~2000) extent at ~4 ky B.P. (fig. S3) "

As you say, but the point I'm making is the model output is showing a sharp downturn in present times, quite unlike any other departure from the long-term trend. If the point you are trying to make is that current sea ice behaviour is nothing unusual, then the opposite appears to be the case based on the information provided here (study is paywalled).

Coincidental to current conversations about milennial climate reconstructions, it would appear that the sea ice profile is consistent with the handle of the hockey stick, and the sharp uptick in temps in present times. But it's only regional and only a model.

Have you read the paper? I'm curious to know what other forcings applied re the top graph, besides orbital forcing. There is such a discrepancy between sea ice concentration levels 8k years ago. I'm guessing they ran the models backwards through time if the main variable parameter was GHG forcing. I imagine the error bars for the different runs would be very wide far back in time.

This paper is based on proxies and therefore it is only appropriate to compare the same proxy with itself. The authors did so and explicitly state, "our records would correspond in the model to an Arctic Ocean sea-ice cover in summer at 8 ky B.P. that was less than half of the record low 2007 level. The general buildup of sea ice from ~6 ky B.P. agrees with the LOVECLIM model, showing that summer sea-ice cover, which reached its Holocene maximum during the LIA, attained its present (~2000) extent at ~4 ky B.P. (fig. S3)"

There is considerable variance on methods to measure sea ice extent, even with satellites.

Reading comments in that link, the apparent variance was primarily due to Anthony misinterpreting the graphs.

I'm a bit doubtful about the model used in the paper, as it doesn't seem the acknowledge the year on year instability that seems to me to be inevitable with a far lower summer ice minimum, but OK, I haven't read the paper.

In previous Arctic sea ice papers I've read positing comparably reduced sea ice cover, the investigations are usually regional rather than Arctic-wide, and there seems to be a see-saw effect between the Western and Eastern Arctic, where sea ice cover declines in one while increasing in the other. From the full article:

In this exercise, our records would correspond in the model to an Arctic Ocean sea-ice cover in summer at 8 ky B.P. that was less than half of the record low 2007 level. The general buildup of sea ice from ~6 ky B.P. agrees with the LOVECLIM model, showing that summer sea-ice cover, which reached its Holocene maximum during the LIA, attained its present (~2000) extent at ~4 ky B.P. (fig. S3). However, despite the similarities at large scale and the long-term trends between model and observations, the complementarity in sea-ice abundance between East (Ellesmere) and West (Greenland), which is seen especially during the HTM, is not simulated in the climate models. The largest reduction in the EJM is indeed seen in the eastern part of the Arctic in association with an enhanced oceanic circulation and net northward heat transport (4). However, there are no signs in the EJM or in LOVECLIM of a concurrent simulated increase in the West. It has been seen in recent years that there is a strong influence on the sea-ice variability from the large-scale atmospheric flow anomalies and associated wind stress (1, 2, 23, 24), and the importance of wind-stress is also known from basic sea-ice physics. Thus, it is likely that the model deficits are related to a too-weak large-scale AO-type flow response to the orbital forcing during the HTM. Such troubles in reproducing past sea-ice variations may also have an impact on future simulated regional changes using the same models. Therefore, improved understanding of these inconsistencies is important.

According to the article, Arctic Summer temps were 2C to 4C warmer than present. It would seem natural to assume that there was less sea-ice than present. But as the forcing is from insolation rather than GHGs, then Winters would possibly (probably?) have been comparably cooler than present for the refreeze. Winters in the Arctic have been getting warmer faster than Summers over the last half century.

The supplementary material outlines forcing variables and provides references for more detial:

Starting from quasi-equilibrium states obtained for 8 ka BP, the model is driven by orbital and greenhouse forcing over the period 8 ka BP-1 AD as in (13). From 1 AD to 2000 AD, volcanic solar and anthropogenic forcing is also applied (12).

I should add that the title of this article: "Paper finds Arctic sea ice extent 8,000 years ago was less than half of the 'record' low 2007 level" is likely to mislead. This result is derived from a model that the authors consider to have failed to model regional variability in arctic sea ice. From their abstract:

Multiyear sea ice reached a minimum between ~8500 and 6000 years ago, when the limit of year-round sea ice at the coast of Greenland was located ~1000 kilometers to the north of its present position. The subsequent increase in multiyear sea ice culminated during the past 2500 years and is linked to an increase in ice export from the western Arctic and higher variability of ice-drift routes. When the ice was at its minimum in northern Greenland, it greatly increased at Ellesmere Island to the west. The lack of uniformity in past sea-ice changes, which is probably related to large-scale atmospheric anomalies such as the Arctic Oscillation, is not well reproduced in models. This needs to be further explored, as it is likely to have an impact on predictions of future sea-ice distribution.

I don't think the title is misleading and is based on a direct quote of the authors. Do you really expect a proxy based on driftwood to be regionally specific within the Arctic? Even state-of-the-art climate models based on the best instrumental data now available are unable to reproduce "large-scale atmospheric anomalies such as the Arctic Oscillation."

To put it accurately, the paper finds that model runs of Arctic sea ice produce half the summer coverage 8k yr BP than at 2007, but that the models have problems because they fail to produce the areas of sea ice increase that appear in proxy records. They are saying that there is a problem with sea ice modeling because it does not represent this variability. It's pretty clear from in the abstract and again in the final paragraph. I'll cite for emphasis:

the complementarity in sea-ice abundance between East (Ellesmere) and West (Greenland), which is seen especially during the HTM, is not simulated in the climate models.... Such troubles in reproducing past sea-ice variations may also have an impact on future simulated regional changes using the same models. Therefore, improved understanding of these inconsistencies is important.

The last two sentences of the abstract summarizes the conclusion:

The lack of uniformity in past sea-ice changes, which is probably related to large-scale atmospheric anomalies such as the Arctic Oscillation, is not well reproduced in models. This needs to be further explored, as it is likely to have an impact on predictions of future sea-ice distribution.

An accurate title would be, "Paper finds models poor at reporducing past sea ice behaviour." They are certainly not claiming that the Arctic summer sea ice cover was half that of 2007 8k yrs ago. That recommend that this outcome is the result of a flaw in the modeling.

"Do you really expect a proxy based on driftwood to be regionally specific within the Arctic?"

The paper is regionally specific. First sentence of the abstract:

We present a sea-ice record from northern Greenland covering the past 10,000 years.

The driftwood proxies are the basis for the notion that there are sea ice increases in other parts of the Arctic 8k yrs ago, concurrent with the significant decrease north of Greenland that is the central feature of the study. The models fail to capture the regional varability that the driftwood proxies suggest.

There is little doubt that there was reduced ice cover in the early Holocene, and it is possible, but by no means certain, that the Arctic had ice-free summers back then, when summer insolation was 8% greater than today. But I'd look to the proxy records rather than the models to make that case.

The first paper you cited just above is a good round up of the knowledge of early holocene sea ice cover at the end of 2010. From the abstract:

The combined sea ice data suggest that the seasonal Arctic sea ice cover was strongly reduced during most of the early Holocene and there appear to have been periods of ice free summers in the central Arctic Ocean.

You get the sense that most of the Arctic was reduced in sea ice cover in the early holocene, but some regions within that expanse may have seen increases.

If, as the proxy records indicate, a greatly reduced ice cover (potentially ice-free) is concurrent with temperatures that are about 3C warmer than now, then in 50 years time we may see conditions similar to those that existed when insolation in the Northern hemisphere peaked 8 - 10k yrs ago. The proxy records show a relatively steady decline in Arctic temperatures following the holocene thermal maximum, consistent with orbitally forced changes (decreasing NH isolation). Something unusual is happening over the last few decades - unusual for its bucking the trend from orbital forcing, and for the sheer pace of the change. And there is no doubt whatsoever that the modern sea ice retreat is pan-Arctic.